The present invention relates to getter devices and, more particularly, to a method for forming supported thin layers of non-evaporable getter (NEG) material and the getter devices formed by this method.
Non-evaporable getter (NEG) materials have been used for the past thirty years in devices in which a vacuum must be maintained for proper operation such as, for example, lamps and evacuated insulating jackets of thermos devices. The most common NEG materials are metals such as Zr, Ti, Nb, Ta, V, and alloys thereof which include at least one other element. For example, commercially available NEG materials produced by SAES Getters S.p.A. of Milan, Italy, include the alloys sold under the trade names St 101.RTM. and St 707.RTM.. The St 101.RTM. alloy has a composition of 84 wt % Zr and 16 wt % Al. The St 707.RTM. alloy has a composition of 70 wt % Zr, 24.6 wt % V, and 5.4 wt % Fe.
In recent years, the importance of planar manufacturing technologies, by which microelectronic devices are produced on substrates generally made of silicon by depositing and selectively removing layers of materials having different electrical properties, has increased. The typical thickness of these planar devices is on the order of a few tenths of a micron. The planar manufacturing operations used to produce microelectronic devices are relatively easy to automate and yield high quality devices. As a result, such planar manufacturing technologies are driving the "planarization" of manufacturing processes in other fields such as optoelectronics and miniaturized mechanical devices. Examples of developing products reflecting this trend include flat panel displays, which may be either the vacuum type or the type with plasma inside referred to as "plasma displays," and so-called "micromachines," i.e., micromechanical devices such as, for example, car accelerometers manufactured by the same techniques used in the field of microelectronics. For devices in which a vacuum is needed, this trend toward planarization requires the development of planar getter devices.
A planar getter device is generally formed by depositing a layer of particles of NEG material deposited onto a suitable carrier, typically a metal sheet. A getter device of this type must have a particle loss as low as possible, preferably zero, as well as excellent values of gas sorption rate and gas sorption capacity. These properties are difficult to obtain simultaneously because the adhesion of the particles of NEG material to one another as well as to the substrate is typically enhanced by sintering heat treatments at high temperatures, which generally impair the porosity of the layer and hence at least its sorption rate.
Supported planar NEG devices may be manufactured by, for example, cold lamination of powders onto a supporting metal tape, as disclosed in U.S. Pat. Nos. 3,652,317, 3,856,709, and 3,975,304. One of the problems with this technique, however, is that the thickness of the deposit is limited to the average size of the particles of NEG material. Moreover, should the NEG material have a hardness comparable to or lower than that of the substrate, the pressure exerted by the compression rollers causes a distortion of the particles which decreases the surface area and therefore the gas sorption efficiency.
Planar getter devices also can be manufactured by electrophoresis, as disclosed, for example, in U.S. Pat. No. 4,628,198. The primary disadvantage of this technique, however, is that layers of NEG material can be formed without difficulty only up to a thickness of about 50 .mu.m. Thicker deposits require long times which are impractical from an industrial point of view. Furthermore, in the electrophoretic technique, the particles are deposited onto the substrate from a liquid suspension and are moved in a charged state by an applied electrical field. A few interesting NEG materials, such as the previously described St 707.TM. alloy, are difficult to electrostatically charge, which makes it difficult to manufacture getter devices including such materials by this technique.
Another technique for producing planar getter devices involves the spray of a suspension containing getter material particles onto a substrate, as disclosed in Patent Application WO 95/23425. When a deposit is produced in this manner, however, a significant amount of the suspension is atomized outside the substrate and, consequently, is lost.
In view of the above, what is needed is a method for forming a supported thin layer or film of getter material having excellent gas sorption and powder loss properties.